1. Field
The present invention relates to technology for data storage.
2. Description of the Related Art
A variety of materials show reversible resistance-switching behavior. These materials include chalcogenides, carbon polymers, perovskites, and certain metal oxides and nitrides. Specifically, there are metal oxides and nitrides which include only one metal and exhibit reliable resistance switching behavior. This group includes, for example, NiO, Nb2O5, TiO2, HfO2, Al2O3, MgOx, CrO2, VO, BN, and AlN, as described by Pagnia and Sotnick in “Bistable Switching in Electroformed Metal-Insulator-Metal Device,” Phys. Stat. Sol. (A) 108, 11-65 (1988). A layer of one of these materials may be formed in an initial state, for example a relatively low-resistance state. Upon application of sufficient voltage, the material switches to a stable high-resistance state. This resistance switching is reversible such that subsequent application of an appropriate current or voltage can serve to return the resistance-switching material to a stable low-resistance state. This conversion can be repeated many times. For some materials, the initial state is high-resistance rather than low-resistance.
These reversible resistance-switching materials are of interest for use in nonvolatile memory arrays. One resistance state may correspond to a data “0,” for example, while the other resistance state corresponds to a data “1.” Some of these materials may have more than two stable resistance states.
Non-volatile memories formed from reversible resistance-switching elements are known. For example, U.S. Patent Application Publication 2006/0250836, filed May 9, 2005 and titled “REWRITEABLE MEMORY CELL COMPRISING A DIODE AND A RESISTANCE-SWITCHING MATERIAL,” which is hereby incorporated by reference herein in its entirety, describes a rewriteable non-volatile memory cell that includes a diode coupled in series with a reversible resistance-switching material such as a metal oxide or metal nitride. Such memory cells can be programmed by applying one or more programming signals to cause the reversible resistance-switching to change from a low resistance state to a high resistance state, which may be referred to as RESETTING. Similarly, the memory cells can be programmed by applying one or more programming signals to cause the reversible resistance-switching to change from the high resistance state to the low resistance state, which may be referred to as SETTING.
However, operating memory devices that employ reversible resistance-switching materials is difficult. Specifically, programming the memory cells can be challenging. It may be desirable to RESET (or SET) the memory cell using relatively few programming signals to save time and/or power. However, the conditions needed to RESET (or SET) can vary from one memory cell to the next. Therefore, it can be difficult to efficiently program all of the memory cells using the same programming conditions.
Also, it is desirable to program a large number of memory cells in a short amount of time. In other words, a high programming bandwidth is desirable. However, it can be difficult to achieve a high programming bandwidth while staying within a maximum current and/or power consumption of the memory device.